Production of alkali metals



March 18, 1941.

H. N. GILBERT V PRODUCTION oF ALKALI METALS `2 Shets-Sheejz 2 Drignal Filed Hatch l1. 1936 INVENTOR. HAR VEY @LBL-ADT ATTORNEY able for trated. Fig. 1 is a vertical sectional view of a two-cell system embodying my invention. Figs. 2 and 3 are sections at right angles to the section of Fig. l, of parts of the apparatus illustrated in'Fig. 1.

The drawings illustrate a two-cell system suitelectrolyzing sodium chloride to produce chlorine and metallic sodium in accordance with my invention. This apparatus comprises a cold cell in which an aqueous salt solution may be electrolyzed anda hot cell in which a molten sodium compound or a fused mixture of sodium compounds may be electrolyzed. The cold cell is shown in section at the left hand side of Fig. 1 and by Fig. 2, which is a section in plane AA of Fig. 1. The shell or container I, which maybe constructed of concrete or other suitable material, has three semicircular wells 2 cast in the bottom thereof. Three vertical wheels 3 are mounted on a rotatable shaft 4 supported by bearings 'I in such manner that the lower part of the wheels 3 depend into the wells 2. The upper part of the cell is covered with a dome shaped gas collector 3 which is made of material resistant to chlorine, preferably ceramic material. Through openings in the top of dome 8 are suspended four vertical anodes 3, preferably made of graphite. The anodes 9 are connected to an electrical lead or bus bar II). Outlet pipe II, made of chlorine-resistant material is provided in the top of dome 8 to condut evolved gas from the interior of the cell. The upper portion of the cell is provided with inlet pipe I2, for introducing an aqueous electrolyte into the cell and outlet pipe I3 for removing aqueous electrolyte from the cell. By means of suitable electrolyte reservoir and pump arrangement not shown, the aqueous electrolyte may be circulated through the cell by way o-f pipe I2 and I3. The shaft 4, upon which is mounted the rotating wheels 3, is caused to rotate by means of gears 5 and shaft 6 which is connected to a suitable source of power not shown.

In operating this apparatus in accordance with one embodiment of my invention, the "cold cell illustrated in the drawings is filled with mercury or other liquid metal or alloy up to a point below the lower ends of the anodes 9. The liquid metal electrode must have a melting point below the boiling point of the aqueous electrolyte used, unless pressure is applied; mercury is preferred for the metal electrode. The liquid metal may enter the cell by means of pipe I'I and may be removed therefrom by way of pipes I4 which are connected to the bottom of the wellsI 2. The upper portion of the cell above the liquid metal level is filled with an equeous alkali metal salt solution, e. g. a solution of sodium chloride, preferably to a point somewhat above the tops of the rotating wheels 3. In the preferred operation of this cell, brine is continuously circulated through the upper portion of the cell and simultaneously the liquid metal which may serve as cathode is continuously circulated through the lower portion of the cell by way of pipes I4 and pipeII. Preferably the liquid metal enters at a point near the upper surface of the liquid metal through pipe I1 and is withdrawn from the bottom of the cell through pipes I4. This circulation, howevenmay occur in reverse direction from that stated if desired.

In the specific modification illustrated by the drawing in Fig. 1 the cold cell is operated in conjunction with a hot cell" of similar construction and the two cells are connected by a:

suitable arrangement of conduits for circulating the liquid metal electrode through each cell and from one cell to the other. The preferred modication includes a heat exchanger I8, whereby the liquid metal leaving the hot cell is cooled before it enters the cold cell and the liquid metal leaving the coil cell is heated before it enters the hot cell. In addition to the use of the heat exchanger, it is sometimes advisable to provide an additional cooling means for cooling the metal leaving the heat exchanger before it enters the cold cell. Such cooling means (not shown) may be of any conventional design. Referring to Figs. l and 3, the hot cell suitable for the production of alkali metal by electrolysis of the fused alkali metal compound in conjunction with an alkali metal alloy as liquid metal anode will be described. The hot cell is shown in section at the right hand side of Fig. l; Fig. 3 is a cross section of the hot cell on the plane BB shown Vin Fig. 1. The hot cell construction includes a shell or container the lower part of which 23 is of relatively heavy construction, e. g. cast iron or cast steel, while the upper part 24 is of some lighter construction, e. g. steel plate. The lower half 23v of the cell is provided with three depressions or wells 25 the cross sections of which, as shown in Fig. 3, are semicircular in shape. A shaft 2l extends transversely of the cell mounted on bearings 4 I, this shaft being provided with gears 28 and counter-shaft 29, the latter being connected to a suitable Source of power not shown for rotating shaft 2l. Three wheels 26 are attached to the shaft 2l so as to rotate therewith, ,the lower portions of wheels 26 depending into the wells 25. In upper portion of the cell are four metallic cathodes 30, preferably made of steel, which are suspended on the hori- Zontal metallic member 3|. Horizontal member 3l in turn is suspended from bus bar 32 which extends through the dome 33 to a point outside the cell. Collecting dome 33, preferably conl structed of cast steel, is mounted above the cathodes 30 and serves to collect molten metal liberated at the cathodes and rising therefrom; Pipe 36 connected to the top of dome 33 is provided to carry oi the collected molten eathodic product into receiver 31 to which it is connected. Receiver 31 is fitted with values 38 at its lower end, whereby the collected molten metal may be removed from time to time. Pipes 39 connected with the bottoms of the wells 25, together with pipe 22, are provided for circulating molten metal anode through the cell. In the upper portion of the cell is suspended a vertical shield 34, .extending completely around all sides of the cell, spaced a short` distance away from the side walls 4. The shield 34, extends downward to a point just above but out of contact with the level of the liquid metal anode in the cell and is supported at its upper end by means of electrical insulation 35 which in turn is supported by the outstanding flange of the side wall 24. 'I'he entire cell preferably is covered with a layer of heat insulating material 40. The hot cell may be supported by I-beams 42, as shown in Fig. 3. I prefer to provide the cell with a cover 44 to prevent access of air to the electrolyte.

In the operation of the hot cell the lower portion is lled with the circulating liquid metal anode preferably to a point slightly above the axis of the rotating wheels, and space above the liquid metall anode is filled with a molten alkali metal compound or a fused mixture of alkali metal compounds to serve as electrolyte. A

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the liquid metal anode iii-i, it vvas iound that autres? liquid metal alloy of an alkali metal, e. g. sodium amalgam, is circulated through the lower t of the hot celll to serve as iianodo. On passare or the electrical current, alkali metal is dissolved from the anode andliberated at the cathode.. 'lihe cathodic product, which is in the liquid state rises in the collector dome 33 and thence :iiovvs through pipe 30 to receiver tl.. The temperaw ture oi the molten electrolyte may he resulated hy adjusting the how of electrical current, desired, the temperature may additionally he reuulated hy the use ol suitable heating and/or coolmu means in contact with the electrolyte.

'.lo provide for circulatins the liquid metal scrvinp as electrodes in the hot and cold celle, pipe i0 conducts the liquid metal from pipe it in the loottom ci the cold cell to pump i0 which elevates the metal and forces it through the heat eirchanser i0.. From the heat exchanger i0, the liquid metal hows through pipe 2i and enters the bottom oi the hot cell hy Way oi pipes t0.. '.ihe liquid metal leaves the hot ccll hy Way ci pipe ti, near the surface oi the liquid metal in trie hot cell and thence flows by gravity through the heat exchanger i0 and thence through pipe il into the cold cell." .l-leat erru changer it, which is illustrated diaeratic ally in illu, l, may he ol conventional decian. iereierahly, it is. covered with a layer oi heat in sulatins material it.. Likewise, pipe 2i, leal: trom the heat exchanger to the Mhot cell may he covered with heat insulation dill 'lihe purpose oi the shield tt in the hot cell is to prevent the vralls t4 from acting as anode. since `vvalls it are in electrical connection ivith cell. lt vvas round that heat resistant insulatm ine ymaterials which would he suitable inthe construction lor :maintaininaP the liquid metal anode out` of electrical contact with that portion oi' the cell which is in contact tvith the molten electrolyte, tended slowly to `dissolve in the elec-7m -trolyte and thus to contaminate it.. s .was

example, when a sieh iound to he the case, ior oi marhle vaas-used as insulator in contact with the electrolyte, the marble slowly dissolvlns to introduce calcium into the electrolyte which inm creased the viscosity and meltina' point ol the electrolyte and tended to `ivy-nate the cath@ odio product with calcium. By means oi shield til, `which is maintained out ol electrical con tact vvith the side vvalls 2t lluy means ci insulator the anodic effect oi wall it was substantially completely elnated.

.in the operation oi' the hot cell" the concenttration oi allrali metal in the liquid metal anode introduced into 'the cell may vary uit Wide limits. l have roundthat when usine mercury as the liquid metal, there is no advantage in increasing: the alirali metal content oi the anode alcove about 0.2% hy weicht: at higher concen-n trations the viscosity or the amalgam may 'ilecome excessively high.. lin the production oi sodium, i prefer to maintain the sodium conIl centration oi cell at 0.03

the liquid metal entering the hot to 1.0% hy weight.

lin general the operation oi. my improved elec trolytic cell as alcove illustrated is similar, regardless of whether it is used as cold cell -vvhereloy the liquid metal is used as cathode or as a not cell wherein a liquid metal alloy of an alkali metal is used as anode. The principle ol the operation is to rotate the wheels 3 or 26, whereby the surfaces of the wheels continuously in the lower part oi the pass first through the molten metal and then through the electrolyte. For Aexample in the operation of the .cold celii the aqueous electrolyte may be a solution of and the liquid metal cathode may be chiefly mercury or mercury alloy.. As the wheel rotates tnroucn the mercury cathode a thin film oi.' mercury adheres to the surface of the Wheel and this thin nlm oi' mercury ls brought into contact with the aqueous electrolyte by the rotation oi the Wheel., in this manner a relatively large suriace oi mercury is hrouuht into contact vvith the electrolyte and a rapid electrolysis ensues. liberation metallic sodium at the surface of the cathode nini. The sodi alloys With the cathode hlm and as the wheel rotates, the resulting amalgam then comes into contact 'with the mer-- cury or Weak; amalgam which is circulated through the hottom portion of lthe cell.. It should he stated at this point that when the surface oi the Wheels il is melde ol' a metal such as iron or steel with vvhich mercury does not readily alloy or the surface oi which mercury does not readily Wei Vpure mercury vvill not readily adhere to the surface oi the wheel. li have discovered that il the mercury initially contains a small amount ol alirall metal, the mercury will readily ere to an iron or steel surface which is reasonably clean to torni a continuous ulm thereon.. For creampie, l have round that a sodium amalgam containinir not less than hy weight oi" sodi vvct a clean plain carbon steel surface, while amalgam less rich in sodium vvill not.. Metals other than which are suitable tor lace vvlll vary i'or 4did'erent alkali metals.. a7l'he lollovvlnc tabulation shovvs the approidmate minirnum concentrations oi sodium and potassium in the amala'arn required to readily vvet different metallic auriacesf:

Approrimate minimum concentration of alkali Metal surface metal in amalgam Sodium Potassium -*M i Perron; by Percent by wel t t Stainless chrome steel u 0.05 mmh 0. l5 one] metal :m., 0.01 0,10 Carbon steel (S. A. E. 10l0) 0.01 0. 05 Nickel 0. 0l 0. 02

ln practicingF my invention to electrolyze aqueoussoiutlons ol allrali metal salts, l prefer to coat the rotatable Wheel or equivalent means utilized vvith an adherent layer oi the amalgam prior to starting the electrolysis. This is clone to prevent chemical reaction hetvveen the metal sodium chloride dit ` lower portion of the hot cell 4 wheel therein until it is covered with the amalgam. The electrolysis then may be started. By this' means, the entire working surface of the wheel w 1 have a uniform, adherent coating of amalgam which prevents contact of the base metal with the electrolyte during the electrolysis.

If the cell is constructed of metal which readily alloys with mercury, e. g. copper or brass, it will not be necessary to have alkali metal initially present in the liquid metal electrode. However, it is preferable to use iron, an iron alloy (e. g. steels), nickel, or a nickel alloy (e. gfMonel metal) for the wheel construction, since such metals do not readily alloy with mercury and hence resist corrosion by the mercury. Chromium-iron alloys such as stainless steel are suitable for this purpose.

In the operation of a hot cell" in the utilization of my invention the general considerations above recited in regard to the operation o the cold cell will apply. For example by circulating an alloy of sodium and mercury through the to serve as anode and utilizing a molten sodium compound, e. g. NaOH, as electrolyte, metallic sodium will be electrically dissolved from the liquid metal anode and liberated at the cathode 'from whence it rises by reason of its low specific gravity into the upper portion of the collecting dome 33 and hence through apipe 36 into the receiver 31. In the operation of a hot cell of this character it is desirable to maintain as low a temperature as possible because of the tendency for mercury to vaporize at elevated temperatures, because of loss of energy by way of radiated heat at excessively high temperatures and because of damage caused to cell parts by warping and corrosion which is liable to occur if excessive temperatures are used.

The molten electrolyte in the hot cell preferably should have a melting point not higher than about 250 to 300 C., if the liquid metal electrode owing between the two cells is mercury or alkali metal amalgam, since the vapor pressure of mercury becomes excessively high at temperatures much above 300 C. I prefer to use an electrolyte of sufficiently low melting point so that the temperature of the fused electrolyte can be maintained at notahigher than 240 to 300 C. Hereltofore, it has not been practicable to utilize mercury as exchange electrode in a two cell system with fused electrolyte because of the relatively high melting point of the electrolytes which has made necessary the use of temperatures of about 350 C. and. higher. The operator therefore was forced to use molten lead as electrode and this prevented the use oi' a cold celli in conjunction with the hot cell.

As disclosed and claimed in my co-pending application filed of even date herewith, I have discovered that various mixtures of alkali metal hydroxide and alkali metal halides may be prepared having melting points lower than about 300 C. and as low as around 200 C. For example, in the production of sodium by=the doublecell system described above, using mercury as exchange electrode, I may use a mixture of sodium hydroxide and sodium iodide which has a melting point not higher than 300 C. Preferably, I use the eutectic mixture of these compounds containing about 55% by weight of sodium hydroxide and 45% sodium iodide which melts at about 225 C, or an approximation thereof. With this eutectic mixture, the cell advantageously may be operated at a temperature of 240 to 250 C. at which temperature the vapor pressure of Neon (19 maar parano-tmp:

aesinet mercury is only about Melting Eutectic mixture point KOH (72 molar percent)+KI KOH (65 molar peroent)+KBr LiOH $45.5 molar percent)+Lil... LiOH 45 molar pernt)+LiBr LiOH (63 molar percont)+LiCl I have found that such fused mixtures of alkali metal hydroxides and alkali metal halides may be electrolyzed with an alkali metal alloy anode over long periods of time with substantially no decomposition or change in composition to electrolytically dissolve alkali metal from the anode and liberate it at the cathode. a.

The speed of rotation of the electrode wheels in the embodiment of my invention described above may be varied within wide limits and the optimum rate of rotation will depend upon the electrical current density, the nature of the electrolyte, operating temperature, the nature of the liquid metal electrode and the amount of alkali metal in the liquid metal electrode. The only limiting factor in regard to the speed of rotation is that of centrifugal force. That is, the wheels should not be operated at such excessive speed that the liquid film on the upper portion thereon tends to be thrown off the wheel by centrifugal force developed by the rotation. For this reason, the maximum possible speed of rotation may vary, depending upon the diameter of the wheel and also on the viscosity of the liquid metal lm, which viscosity will vary depending upon the amount of alkali metal dissolved in the liquid metal film, the nature of the liquid metal and also to some extent upon the temperature of op... eration. I have found, for example, that a steel wheel three inches in diameter may be operated at a .speed of up to about 100 revolutionsper minute with little or no tendency for the liquid film to be thrown off, even with very low amounts of alkali metal in the lm, the liquid metal being mercury. In general I prefer to operate at much lower speeds, for example, for a three inch steel wheel, using liquid mercury as electrode, I prefer to rotate the wheel at about l to 25 revolutions per minute.

It will be obvious from the above description that numerous modifications of my herein described invention may be made Without departing from the spirit and scope thereof. The principle of my invention is to provide a composite electrode which consists of a solid surface coated with an adherent nlm of liquid metal and means for contacting the solid surface rst with a reservoir of liquid metal and then with an electrolyte, whereby a substantially.continuous film of liquid metal adhering to the solid surface serves as active electrode surface. My invention also includes the application of this principle to a double-cell electrolytic process with a liquid metal exchange electrode for electrolyzing V(1) an aqueous solution with the liquid metal as cathode and (2) a low-melting fused electrolyte with the liquid metal dll bil

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as anode. In the modification described above by way ol illustrating my invention a rotating wheel or disc is utilized for this purpose, the rotation of the wheel serving to coat the solid surface with liquidl metal. contact it with the electrolyte and then again bring it into contact with the reservoir of molten `metal whereby the metal nlm, after it has momentarily served as electrode is wholly or partially replaced by fresh liquid metal. In place of a rotating wheel for this purpose .l may utilize a continuously moving band or belt oi sheet metal, one portion of which lies in a reservoir of liquid metal while another portion thereof lies in the electrolyte. modiiication, if desired, spaced directly above the reservoir of molten metal into which the lower portion of the band is immersed or il desired the electrolyte may be in a container placed oil' to one side of the liquid metal reservoir and on the same level therewith or above it or below it. Still another modification the rotating wheel in acbrought into contact may be accomplished a sheet of steel from a cranlc shaft whereby ou rotation ol the cranlr, the sheet is alternately raised and lowered so that at i'lrst it is dipped into a molten metal reservoir and then may be raised into the supermatant electrolyte. The same edect is also accomplished by providing a wheel of 'radiating spokes in place oi the solidfacecl wheel described above and shown in the appended drawings.

ln still another modincation of my invention, .l may pass a solid metal surface throughliquid metal to piclr up an adherent lilm, contact the nlm-coated surface with the electrolyte to serve as electrode. whereby alkali metal is liberated and alloyed with `the film and finally move the sur- :face with adhering alloy film into contact with a substance reactive with alkali metal (e. g.; water) or utilize the alloy film-covered surface directly as anode in a hot'cell Thus, an endless band of flexible steel or equivalent means may be used to carry a film of mercury or alkali metal amaigain coated thereon through a cold cell as cathode, thence baclr to the cold cell, At a .sultable point,v the band may be passed through a reservoir of mercury to replenish the little lost by evaporation in the hot cell. By this means. the power required to circulate mercury and the total amount of mercury required is still further diminished. l

lVarious other means of alternately coating .metal surfaces ad bringing them into contact with electrolyte will be obvious to the skilled rnechanic. ll prefer, however, to use the herein described rotating disc this manner efficient transfer of a film of molten metal from molten metal reservoir to electrolyte with the electrolyte. This .may be obtained with the least complexity of Inov-l ing parts, the simplest type of cell design and greatest overall efficiency. Also I have found that the greatest cell eiiiciency is obtained when the rotating wheel is in the form of a relatively thin disc, so arranged that at least the major portion of both of the plane sides of the disc dip into the mercury or amalgam in the manner shown in the appended drawings. This arrangement provides a larger active surface, as compared with a rotating cylinder, where only one side of the for example by suspending i'or this purpose, since in cylindrical surface is utilized., Also, in my pre.. ferred modification, using the rotating discs. the amalgamated surface acting electrode forms an extended vertical electrode surface: this is the best arrangement, since it brings the electrode surface into contact with electrolyte substantially throughout the electrolyte and permits electrolysis products, e. g. gases or molten light metals, to readily and quiclrly pass upwardly out of the gone of electrolysis.

The utilization of my above described liquid metal iilm is not confined to electrolytic proccirculating sodium amalgam through an apparatus having a reservoir for the amalgam and ro tating discs or their equivalent dipping into the may be circulated or replenished as desired.A a similar manner a liquid alkali metal alloy may be brought into contact with a gaseous substance reactive with the allrali metal..

lldy herein described method and apparatus is especially well adapted for the production of althis purpose, ll may employ an electrolytic cell cold cell shown in ligs. l and 2 to produce 'the allrall metal amalgam. For re acting the amalgam with water, l may employ a decomposition cell similar to either ci' the cells shown by the drawings or a cell oi simpler de sign, which preferably is constructed oi iron or steel. Such decomposition cell would be similar per portion extending into water or dilute caustic solution. Preferably, graphite electrodes, similar to the cathodes ll of l'igs. l and 3, are arranged in close juxtaposition to the portions of the discs in the aqueous medium.. These graphite electrodes are electrically connected with the amai gam in. the well or wells, for example by being loined to the walls of the cell, which may be' made of steel. The amalgam is circulated between the two cells in the manner illustrated by Fig.. l, ercept that `the heat exchange" il may be eliminated. Water is circulated through the upper portion of the decomposition cell, and the resulting caustic solution leaving the cell may be evaporated to recover the solid allrali metal hydroirw ide. Since the decomposition cell may be operated at ordinary temperatures (e. g. l5 to 30 ci.) the cell need not be insulated.. 'Hydrogen evolved by the reaction may be allowed to escape into the air or may be collected by means of a suitable collecting dome, e. g. one similar to dome di of er eftlciency.

Also, my invention is not restricted to utilizations employing mercury as a carrier for alkali metals, but is also applicable to the use of other liquid metal electrodes, e. g.. molten lead, lead alloys, alloys of bismuth, tin, lead and cadmium, such as Woods metal" and the like.

A further advantage and utilization of the herein described inventinn memes :n +1 mi hill dit

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tion of the moving surfaces coated with adherent liquid metal film as a heat exchangel device. For example, in the use of the rotating disc composite cathode described above for electrolysis of a salt solution, the amalgam-coated rotating disc effectively acts as a heat exchange means to extract heat from the solution undergoing elecerations ordinarily there is more or less heat evolved by the reaction, and by. means of the apparatus and method described herein the reaction heat is effectively removed, especially in the case of reaction with liquids such as Water, with the result that the reacting liquid is at all times maintained at substantially the same temperature as that of the underlying mass of amalgam. In the case of endothermic reactions, by supplying heat to the underlying reservoir of liquid metal or amalgam, heat is eiectively supplied to the reacting liquid by the heat exchange effect above described. In either case, the reaction vtemperature may be controlled within narrow limits by this means.

My invention is also applicable to heat exchange operations in the absence of electrolysis or other chemical action. For example, a liquid may be effectively cooled by use of the herein described rotating bodies having an adherent liquid metal coating, which bodies are partially immersed in a reservoir of the metal and extend into the liquid to be cooled. By supplying heat to or extracting heat from the underlying reservoir of liquid metal, the temperature of the liquid to liquids reactive with be cooled or heated thus may be controlled within very narrow limits. If desired, liquid alkali metal amalgams containing suillcient alkali metal to cause the amalgam to adhere to the metallic surface may be utilized in such heat exchange operations. Such adaptations may be used for example to cool liquids which are not reactive with alkali metals (e. g. hydrocarbons) when it is desired to carry on the heat exchange operation without chemical reaction or without destruction of the alkali metal utilized. This heat exchange method and apparatus also may be used to add or subtract heat from reactions in which there are no reactants or products which are reactive with the alkali metal amalgam.

I claim:

1. An electrolytic cell comprising .fa container, having electrically conductive interior side walls, a body of liquid metal adapted to serve as an electrode in the lower portion of said container, an electrolyte floating on the surface of said liquid metal and a metal shield within said container and spaced from the side walls thereof extending through said electrolyte close to but out of contact with said liquid metal and electrically insulated from said side walls.

-2. An electrolytic cell comprising a container having interior side walls of metal, a liquid metal anode in the lower portion of said container, an electrolyte floating on the surface of said liquid metal anode, and a metal shield close to but electrically insulated from said side Walls, which shield extends through said electrolyte close to but out of contact with said anode.

3. An electrolytic cell comprising a container having interior side walls of metal, a liquid metal anode in the lower portion of said container, an electrolyte oating on the surface of said liquid metal anode, and a metal shield close to but electrically insulated from said side walls, which shield extends through said electrolyte close to but out of contact with said anode and at least one rotatable disc so positioned in the cell with substantially horizontal axis that it lies partly in said electrolyte and partly in said liquid anode.

HARVEY N. GILBERT. 

